The default value depends upon the character height of the current font.
If the control's C1.Win.C1List.ListBase.C1ListBase.AllowRowSizing property is set to True and the C1.Win.C1List.ListBase.C1ListBase.SelectionMode property is set to CheckBox, then the user can adjust the C1.Win.C1List.ListBase.C1ListBase.ItemHeight property at run time by dragging the row divider between any pair of checkboxes.